1. Technical Field
This invention relates to fuel cell systems, and more particularly to a procedure for shutting down or starting up a fuel cell system.
2. Background Information
Fuel cell systems, including the catalytic components of a fuel processing system for converting organic fuel to hydrogen, and especially the anode side of the fuel cell itself, generally require purging upon shut-down to remove residual hydrogen, and purging upon start-up to remove air. This is necessitated for several reasons, including: The elimination of the potential for flammable mixtures of hydrogen and oxygen; minimizing performance degradation of fuel processing system catalysts and electrode catalysts; and prevention of hazardous material formation during the start-up and shut-down process. Common practice is to purge components with inert gas such as nitrogen or nitrogen mixed with other gases harmless to the component being purged. For example, U.S. Pat. No. 4,537,839 describes using inert gases (defined therein as gases xe2x80x9csubstantially free of hydrogenxe2x80x9d), such as product gases from a catalytic combustor, to purge a fuel cell. U.S. Pat. No. 5,248,567 also describes the use of a fuel cell purge gas from which the combustion elements (mainly oxygen and reactive carbon) have been removed.
In fuel cell power plants it is also known to use a truly inert gas, such as nitrogen, to purge the anode side of the fuel cell and to purge components of fuel processing systems that convert organic fuels, such as gasoline or natural gas, to hydrogen. It is desirable to avoid the need for and cost of having a tank of inert gas, such as nitrogen, as part of the fuel cell system, as well as the need to continually replenish that inert gas.
In the present invention, organic fuel and air is provided to a burner of a fuel cell system to produce an inert gas for purging the fuel cells and/or for purging fuel processing system components, such as a reformer and shift converter. The fuel cell anode flow fields may be purged of air upon start-up prior to providing reactants to the cells. Upon shutdown, the inert gas may be used to purge the cells of reactants and to purge fuel processing system components of residual hydrogen and carbon monoxide. As used in this paragraph and hereinafter, including the appended claims, xe2x80x9cinert gasxe2x80x9d means not only truly inert gases, such as nitrogen, but a gas that only contains constituents that are not harmful to any fuel cell system components being purged. Preferably, the inert gas contains essentially only nitrogen, water vapor and carbon dioxide.
In one embodiment of the present invention, a fuel cell system that includes a reformer for converting an organic fuel to hydrogen is shut-down by disconnecting the fuel cell from its load, halting the conversion of organic fuel to hydrogen, directing the organic fuel and air into a burner to produce an inert gas, and passing that inert gas through the reformer to purge the reformer of residual hydrogen and carbon monoxide.
In another example, in addition to purging the reformer, the inert gas from the burner may also be passed through the anode flow field of the fuel cell to purge the anode flow field of residual hydrogen upon shut-down. The purging of the reformer and the anode flow field may be done with separate, parallel flows of the inert gas from the burner, or the inert gas may be passed in series, first through the reformer and thereafter through the anode flow field.
If the fuel processing system includes other catalytic fuel processing components, the inert gas from the burner may be passed in series through all of those components to purge the entire fuel processing system of residual hydrogen and carbon monoxide.
Once the purging is complete, the flow of organic fuel and air to the burner is halted. During continued shutdown, air eventually fills the volumes of the fuel processing system components as well as the anode and cathode flow fields. Upon start-up of the fuel cell, but before connecting the fuel cell to the load, air and organic fuel may again be directed into the burner to produce the aforementioned inert purge gas. That purge gas may then be directed through the anode flow field to purge it of air. Once such purging is complete, organic fuel flow to the inert gas generating burner is stopped, and organic fuel is fed directly into the fuel processing components to begin the conversion of that fuel to hydrogen. Reactants are then provided to the fuel cell and power production is commenced.
In the case where the fuel processing system includes a shift converter that uses a nickel or nickel alloy catalyst, care must be taken not to conduct the purging of that component at temperatures between about 90xc2x0 C. to 200xc2x0 C. Between those temperatures, nickel reacts with the carbon monoxide in the fuel-processing stream to form nickel carbonyl, which is volatile and toxic. Shift converters that use noble metal or noble metal alloy catalysts would not have this problem.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.